Persistent volumes under taskgroup non-root user may not be writable if executor user under root.

If the executor and the task are with different users, the persistent volume may not be writable. For example, in the case of the default executor consuming persistent volumes, if the executor is root user (from the frameworkinfo) and the task is non-root user (from the commandinfo), the persistent volume would be owned by the root which is unwritable for the task.

This is caused by the persistent volume support for nested container with the default executor is a workaround (rely on the default executor specifying a sandbox_path volume). We should figure out a correct way to support persistent volume primitive for nested containers.